Do we know that the developers didn't confirm this hack with the hardware people?
It could be "defined by e-mail".
It could be "defined by e-mail".
Of course, if you don't have cloners immediately nipping at your heels, you might end up releasing a smash hit that takes advantage of UB in a way that forces you to define the behavior as "it works however it needs to work to keep that game working", because eventually you yourself (the original console manufacturer) will tape out later revisions of the CPU, and you'll want to make sure they can run that game, given that it's "officially licensed" by you.
If you had the right static analysis tools in play, though, you might have wanted to run them over the game, notice the use of UB, and thus fail the game at the QA stage, before things need to escalate to that point.
What NES cartridge from 1986 can you plug into current Nintendo boxes?
Even though the 1CHIP SNES is a complete ground-up re-layout (has to be, turning a bunch of individual chips into one SoC), every (officially licensed) cartridge made for "the SNES" works on a 1CHIP SNES.
A hardware revision like this, requires retaining bug-for-bug UB compatibility with officially-sanctioned released game titles, because to do otherwise is to doom the console's support line to endless complaints from people whose games don't work. But hardware revisions are not concerned with keeping UB working the same where no officially-sanctioned released title took advantage of said UB. For everything not constrained by bug-for-bug compat with an existing title, all that's required of the new revision is that it keeps to the spec.
If you were a third-party in the middle of developing a new title, and were relying on some new UB you found, when suddenly your console mfgr released a new revision, it could turn out that your clever UB hack won't work the same on the new revision. (And this happened; it was why unlicensed titles—your Game Genies, your Aladdin Deck Enhancers, etc.—frequently wouldn't work with later console revisions.)
The one playing semantics is you, since your apparent concern is the definition of the term "undefined behavior" and whether something falls under that definition.
My point is about how well an engineering decision is justified, not what term applies to it according to some document.
I hadn't put thought into how important that scroll effect is to the game, but if there was a clean wipe between scenes it would have been tremendously distracting. This technique really is essential to the feeling of immersion.
You are correct thought, often guarantees about behavior can be made after the fact. Generally what happens these days is someone determines they need or want to use a certain undefined behavior. The hardware people then go, look at the RTL for the chip and confirm it behaves in a certain way, and then they UPDATE the documentation to explicitly document the newly guaranteed behavior.
Conversely, often something that is documented to work just doesn't. Especially in the case of console developers who tend to be using early steppings of custom silicon. In that case sometimes when you go to the HW team they fix it in the next stepping, sometimes they document it as errata and move on.